The invention is generally related to packet-oriented or cell-oriented processing of data units in packet or cellular data transmission, and especially to the cell-oriented identification of ATM cells for keeping the cell flow consisting of them in sequence.
The ATM (Asynchronous Transfer Mode) is a strong candidate for a fast future communication protocol, for example, in B-ISDN networks (Broadband Integrated Services Digital Network) and generally in communications between data trans-mission devices. The network consists of nodes and terminals, and of links between them. In an ATM network, data is transferred as cells in digital form, each cell comprising a so-called payload of 48 bytes and a header of 5 bytes. In order for the amount of the header information to be kept at a minimum, the headers do not contain complete routing information between the transmitting and the receiving devices but only information on the virtual path and channel in which the respective data transfer connection is carried. The nodes of the network contain the necessary routing information on the basis of which the respective identifiers of the virtual path and channel are interpreted as a reference for the next respective node.
The use of the cell header is defined in the specifications of the ATM system. The header contains a Virtual Path Identifier (VPI) of 8-12 bits, a Virtual Channel Identifier (VCI) of 16 bits, a Generic Flow Control Field of 0-4 bits, 3 bits which are used to indicate the type of payload, one bit which is used to indicate the priority rating of the cell, and 8 bits which contain the error correction code calculated on the basis of the rest of the cell header. The cell header in particular does not contain any information that individually identifies the cell in question, hence the success of the data transmission depends on maintaining the relative order of the cells. The ATM as such does not contain mechanisms for correcting errors caused by the loss of cells or by misordering. Errors on the cell level cause a Protocol Data Unit (PDU) containing several cells and defined on a higher level to be detected erroneous and discarded, whereby it might be necessary to retransmit a large number of correct cells in addition to the incorrect ones.
Traditionally, ATM-links are perceived as wired circuits or optical cable connections, whereby the above-mentioned requirement for maintaining the relative order of the cells has not caused major problems. However, it is anticipated that the terminals of future data transfer solutions must have the same kind of mobility and independence of permanent connections as mobile phones of cellular networks already have.
Because of its effectiveness and the provision to flexibly modify it, the cellular radio network is a likely architecture in the future. A typical cellular network comprises several base stations (BS) which are affiliated with--possibly through a base station controller (BSC)--a mobile switching center (MSC). In a large cellular network there are numerous mobile switching centers as well as base stations and base station controllers that work under the mobile switching centers.
A typical phenomenon of cellular systems is the change of base stations, i.e., the handover, where a given mobile terminal moves from the transmission range of a first base station to that of a second base station, whereby essentially all the data transmission between the terminal in question and the network is rerouted through the new base station. A perfect synchronization of the operation during the handover is often not possible, which may result in packet loss, duplication or misordering. This is especially severe in the ATM system where a successful transmission of all the cells and ordering are a prerequisite for effective data transmission.
The article "BAHAMA: A Broadband Ad-Hoc Wireless ATM Local-Area Network", Proc. ICC '95, Jun. 18-22 1995, Seattle, written by K. Y. Eng et al., discloses a method in which the GFC field in the header segments of ATM cells is used to implement cell-oriented sequential numbering. The purpose is to contribute to the synchronization and combination of cell flows that arrive at a given conjunction point along two parallel routes. The purpose of numbering the cells is aimed particularly at identifying them unequivocally, so that cells are not duplicated or lost when the cell flows are combined and that their order will remain the same. In this case, a problem might occur because only numbers from 0 to 15 can be presented in the GFC field with a maximum of four bits, whereby the numbering cycle remains so short that cells with the same number belonging to subsequent cycles may become misordered.